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Operational Minimization

Currently, the most effective method of reducing bat fatalities at wind turbines is to employ a strategy known as operational minimization, alternatively known as curtailment. This strategy limits blade rotation, the primary cause of bat fatalities, during high risk periods (e.g. low wind, fall migration period). Bat Conservation International, under the auspices of the Bats and Wind Energy Cooperative (BWEC) was the first to test this strategy in the U.S., demonstrating that bat fatalities could be reduced up to 93 percent. Estimated loss of wind generation was 1–3% of the annual power production. The actual power loss can vary by site and effect power purchase agreements, that is why BCI and others are investigating ways to optimize this strategy to make it more economically viable, and developing alternative strategies, such as Ultrasonic Acoustic Deterrents

Operational Minimization: A twofold strategy of feathering turbine blades (i.e., positioning the blades parallel to the wind) and raising the wind speed at which the blades begin spinning and generating electricity (aka the cut-in speed) to slow rotation at lower wind speeds.

Raising the cut-in speed (the wind speed at which the spinning turbine blades begin to generate electricity) by 4.9 to 9.8 feet per second (1.5 to 3.0 meters per second) above the manufacturer’s preset speed is an effective impact reduction strategy because most bat fatalities occur at relatively low wind speeds. Thus, keeping the blades from spinning until wind speed reach 16 to 20 feet per second (5.0 to 6.0 meters per second) can significantly reduce bat fatalities for relatively little loss in power generation - since at many sites the majority electricity is generated at higher wind speeds.

There is even an opportunity to reduce bat fatalities with little to no loss in power. By simply feathering the turbine blades (pitching them parallel to the wind so they are moving slowly) below that preset speed, bat fatalities can be reduced by an average of 35 percent. In fact, this is a supported best management practice by the American Wind Energy Association (AWEA) and a mitigation measure in the World Bank’s Environmental, Health, and Safety Guidelines for Wind Energy

A great deal of the work BCI has been pursuing in the wind arena in recent years requires a better understanding of how, why, where, when, and which species of bats are killed at wind facilities, with the aim of a more precise operational minimization strategy that has minimal impact on electricity generation.

Wind facility at sunset Photo Courtesy of Michael Schirmacher

Cris Hein, Director of BCI’s wind energy program, said he thinks it’s very possible to devise even more specific operational minimization recommendations. But it requires much more information—including details like the exact time of night bats tend to interact with wind turbines, weather data, such as temperature and barometric pressure, and exact wind speeds and direction.

"In my mind, you couldn’t ask for an easier fix,” Hein said. “In most situations, it’s only required at night, under certain wind conditions, and for a couple months out of the year. Even at a core scale, that’s pretty well refined. But we’re trying to see if we can narrow it down even more, by looking at other variables the industry could use to reduce the conditions under which curtailment should be used.”

“We do have a collaborative relationship with the industry, and this is an opportunity to work with them to find a solution,” Hein said.

However, currently only a few wind energy facilities implement operational minimization unless they developed Habitat Conservation Plans with the U.S. Fish and Wildlife Service. Though an effort by the wind industry and the U.S. Fish and Wildlife Service to develop a multi-state Habitat Conservation Plan covering eight states in the Midwest is currently under review.

“We have the goal of working collaboratively to resolve this issue that meets everybody’s needs,” Hein said. “Not only BCI’s goals of protecting bats, but also in being able to generate renewable electricity. But until we can find a solution that’s accepted by the conservation community and federal agencies, and is cost-effective enough for the industry to buy into, we have our work ahead of us.”